Dr Hay was concerned about the acid from fruit and from vinegar.
Rhubarb and cooked tomato were also reputed to be excessively acid.
Modern methods of food analysis and a much greater understanding of the process of digestion and absorption of food can help clarify the position.
Fruit acids should not be a problem, Fresh acid fruit are an essential part of a balanced and healthy diet. They should be eaten on their own or as part of a protein meal.
Vinegar is a weak solution of ethanoic acid and this is metabolised to carbon dioxide and water. The carbon dioxide is eliminated with exhaled air and places less of a burden on the digestive system than alcohol. Vinegar should be eaten with protein rather than with starch, as it will prevent the action of the starch enzymes in the mouth. The slight acidity will also preserve the vitamins in a protein meal. Lemon juice can be used in place of vinegar in most recipes.
Similarly, cooked tomato should not be a problem as part of a protein meal, and it will also help preserve the vitamin content of the meal.
Rhubarb contains oxalic acid. In early spring when the amount of oxalate in the stem is low, rhubarb can safely be eaten, but by the end of May, the concentration is starting to rise and rhubarb should be avoided. The oxalic acid also prevents the absorbtion of calcium from food and so should not be eaten on a regular basis. The leaves of rhubarb should never be eaten.
Far more dangerous to eat are foods made from mixtures of fat and sugar such as biscuits and cakes.
Fat and sugar mixtures are a double danger to our bodies. Because the ingredients are highly refined, we don't know when we have eaten enough. The refined fats are absorbed quickly into the bloodstream where they coat and clog blood vessels, and have to be stored quickly in fatty tissue. The sugar has equally serious effects on the body's mechanisms. Almost all medical opinions and government health guidelines now agree that sugar and fat mixtures make a major contribution to heart disease and cancer. Avoid them!
Chocolate is also a fat sugar mixture that includes traces of drugs that can cause cravings and addiction. The amount of chocolate in the diet should severely restricted.